1. Field of the Invention
The present invention relates to a measuring instrument for overlay registration,and more particularly to a method for checking the accuracy of a measuring instrument for overlay registration.
2. Description of the Prior Art
Photolithography processes are crucial steps in fabricating Integrated Circuits and transferring a pattern on a mask to a photoresist layer atop a wafer. Photolithography processes include carrying out photoresist coating, explosing, and developing steps in sequence in a dark room which can prevent the photoresist from being undesirably exposed. Semiconductor devices are fabricated step-by-step, in general by repeatedly carrying out the steps of forming a thin film, then forming a patterned photoresist layer through photolithography processes, and finally etching the thin film using the patterned photoresist layer as a mask, so that alignment between each step and proceeding step is very important for product yield. It is inevitable that a little misalignment always occurs due to instability in the exposing machine, form example in the stepper mechanism. Therefore, in a practical production line, there are ranges of overlay specifications defined by producers as a misalignment tolerance to determine whether the misalignment measured from a measuring instrument for overlay registration can be accepted or not. In order to get higher throughput and yield, this overlay specification of misalignment tolerance for each exposing step should be well defined, i.e, not too tight and not too loose and differs for each exposing step.
In order to be easily aligned, on a scribing line of a mask there is formed an overlay mark which can help align the mask with the proceeding layer having an overlay mark at corresponding positions atop a wafer by overlaying the central points of these two overlay marks. A pilot run wafer is always made to ensure that a misalignment of this pilot wafer measured from a measuring instrument for overlay registration meets the overlay specification for this exposing step as defined by producers, so that all remaining wafers can be safely released. Otherwise, the pilot wafer must be reworked again including removing photoresist and using additional photolithography processes until its misalignment measured by the measuring instrument for overlay registration meets the corresponding overlay specification defined by producers.
Coventionally, an overlay mark created in a "box within box" form, where the "outer" box is greater than the "inner" box and these two boxes have the same concentric point, is commonly used in photolithography processes, as shown in FIG. 1. However a problem arises that there may be some shift amounts between these two central points. Referring to FIG. 1, a proceeding patterned layer formed by the aforesaid depositing and photolithography processes is represented by the inner box constituted by bars labeled 3,4 and left on a scribing line of a wafer. On the other hand, the outer box consitituted by bars lebeled 1,2 is used to represent a patterned photoresist layer atop the wafer formed by exposing and developing and left on a corresponding position of the wafer. "A" and "B" denote the central point of the inner box and that of outer box respectively and a vector is formed by pointing from "A" to "B" to represent a shift vector of an overlay registration, which has a vector component in the X direction denoted by .DELTA.X and in the Y direction denoted by .DELTA.Y. In other words, .DELTA.X and .DELTA.y can represent the shift amount of central point of the outer box relative to that of the inner box in the X and Y directions respectively. When there is no misalignment i.e. "A" and "B" are the same point, the shift vector of the central point of the outer box relative to that of the inner box should be (0,0). The shift vector measured from a measuring instrument for overlay registration according the aforesaid method is obtained under the condition that the measuring instrument has 100 percent accuracy. If not, the measured shift vector has a larger deviation, thus making the product yield more unstable and lowering the quality of product. In a practical production line, the facility engineers carry out a weekly or monthly periodic maintenance to maintain a measuring instrument for overlay registration by using the following method. They take wafers from practical production line representing four crucial steps in Integrated Processes, i.e. SiN, SiO.sub.2, poly-Si, and metal layers, and then measure their overlay absolute value by the measuring instrument for overlay registration, by which the engineers can make sure that it has good repeatability for each time of measuring. Measuring accuracy of the measuring instrument for overlay registration itself, however, can not be checked by the aforesaid method. Therefore, the shift vector measured by it is unreliable and results in an unstable product yield. Hence, a method needed to provide which allows for checking measuring accuracy of a measuring instrument for overlay registration itself, thereby making the product yield more stable. Since some misalignment may happen during an exposing process, it is also needed to provide the method for checking measuring accuracy of a measuring instrument for overlay registration, which is not affected by accuracy affecting factor coming from an exposing machine such as a stepper.